The present invention relates generally to an improved method for estimating the camshaft phase angle in an engine with variable cam timing.
The advent of variable cam timing in internal combustion engines has complicated the engine management task. Within the engine control unit, the electronic throttle valve position (alternatively, an idle bypass valve opening if not equipped with an electronically actuated throttle valve), fuel injection pulse width, spark timing, position of the exhaust gas recirculation valve, and the cam phase angle are engine variables commanded by the engine control unit to provide the power demanded by the operator of the vehicle while also delivering high fuel efficiency, low emissions, and acceptable drivability. These engine variables are strongly coupled and have a delay time constant associated with them. Thus, the task of changing among operating conditions in a smooth manner is enabled by the engine control unit containing models of the interdependencies among the variables, dynamic models of the various actuators, accurate information from sensors about the status of the various actuators.
The inventors of the present invention have recognized that the accuracy of prior art methods for predicting the actual cam phase angle can be improved. As a result, the coupled parameters, i.e., spark timing, throttle position, etc. listed above, may be computed inaccurately due to being based on inaccurate input cam phase angle data. One prior method relies on the output of a sensor on the cam phaser. Because the signal from the sensor is noisy, the signal is filtered, thereby reducing the bandwidth of the signal and thus, causing a delay. Another prior method relies on a model within the engine control unit and bases the prediction on the commanded phase angle and the dynamic characteristics of the cam phaser. The cam phaser may fail or may change dynamic characteristics over its lifetime causing the prediction to be in error.
The drawbacks of prior art approaches are overcome by a method for determining an estimated camshaft phase angle of increased accuracy by determining a desired camshaft phase angle, determining an observed raw camshaft phase angle, and basing the estimated camshaft phase angle on the desired camshaft phase angle and the observed raw camshaft phase angle. The raw observed camshaft phase angle may be based on the output of a camshaft phase angle sensor located proximately to the camshaft.
A primary advantage of the invention disclosed herein is a prediction of cam angle of increased accuracy and with a lesser delay than prior art methods.
A further advantage of the present invention is that it provides an accurate prediction of cam phase angle even as the cam phaser performance changes due to wear, failure, ambient conditions, or other anomaly.
A further advantage of the present invention is that the prediction of the disclosed method provides a less noisy signal than prior art methods.
The above advantages and other advantages, objects, and features of the present invention will be readily apparent from the following detailed description of the preferred embodiments when taken in connection with the accompanying drawings.